In multi-band loudspeaker systems it is often observed that passive loudspeaker systems lack the sound quality of bi-amped or active loudspeaker systems. It is sometimes also observed that very low impedance loudspeaker cables provide an improvement in sound quality. Many theories are advanced as to why these results occur and in fact it is not uncommon to see engineers attempt to debunk these claims for theoretical reasons without seeing a need to perform a listening test.
However loudspeaker drivers in passive multi-band systems are not “firmly” connected to the low output impedance “controlled output” or feedback point of the amplifier. Instead the drivers are isolated from the controlled output or feedback point to some degree by various impedances, including the loudspeaker cable impedance and the impedance of a damping output inductor. Even when the drivers are connected in a bi-wired arrangement there is still a damping output inductor partially isolating the controlled output of an amplifier from the loudspeaker drivers. The damping output inductor or at least the impedance associated with the damping inductor is one reason why bi-wired configurations do not sound as clean as bi-amped configurations.
In passive systems all drivers connected to a single amplifier act as loads for the back EMF of all other drivers connected to the single amplifier regardless of any electrical filtering interconnecting the drivers. This is especially the case where there are no electrical filters or where the filters are of low order. If the loudspeaker drivers were firmly tied to a controlled output having a low output impedance, driver interaction would be negligible. However in consumer packaged amplifiers they rarely are firmly tied because damping output inductors and/or other components are typically present between the controlled output or feedback point and the amplifier terminals.
Typically in a passive system, the damping output inductor or equivalent circuitry in the amplifier and cable impedances isolate the controlled output or feedback point from a branching point to individual loudspeaker drivers so that back EMF from drivers results in circulating currents between drivers causing audible corruption of the sound produced.
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